diff --git a/yaql/standard_library/queries.py b/yaql/standard_library/queries.py
index a5e6a8b..ace1af2 100644
--- a/yaql/standard_library/queries.py
+++ b/yaql/standard_library/queries.py
@@ -343,12 +343,14 @@ def group_by(engine, collection, key_selector, value_selector=None,
              aggregator=None):
     groups = {}
     if aggregator is None:
-        aggregator = lambda x: x
+        new_aggregator = lambda x: x
+    else:
+        new_aggregator = lambda x: (x[0], aggregator(x[1]))
     for t in collection:
         value = t if value_selector is None else value_selector(t)
         groups.setdefault(key_selector(t), []).append(value)
         utils.limit_memory_usage(engine, (1, groups))
-    return select(six.iteritems(groups), aggregator)
+    return select(six.iteritems(groups), new_aggregator)
 
 
 @specs.method
diff --git a/yaql/tests/test_queries.py b/yaql/tests/test_queries.py
index 57b827e..da65ec3 100644
--- a/yaql/tests/test_queries.py
+++ b/yaql/tests/test_queries.py
@@ -212,18 +212,18 @@ class TestQueries(yaql.tests.TestCase):
         self.assertItemsEqual(
             [[1, 'ac'], [2, 'be'], [3, 'd']],
             self.eval('$.items().orderBy($[0]).'
-                      'groupBy($[1], $[0], [$[0], $[1].sum()])', data=data))
+                      'groupBy($[1], $[0], $.sum())', data=data))
 
         self.assertItemsEqual(
             [[1, ['a', 1, 'c', 1]], [2, ['b', 2, 'e', 2]], [3, ['d', 3]]],
             self.eval('$.items().orderBy($[0]).'
-                      'groupBy($[1],,  [$[0], $[1].sum()])',
+                      'groupBy($[1],,  $.sum())',
                       data=data))
 
         self.assertItemsEqual(
             [[1, ['a', 1, 'c', 1]], [2, ['b', 2, 'e', 2]], [3, ['d', 3]]],
             self.eval('$.items().orderBy($[0]).'
-                      'groupBy($[1], aggregator =>  [$[0], $[1].sum()])',
+                      'groupBy($[1], aggregator => $.sum())',
                       data=data))
 
     def test_join(self):